Axial flow rotary machines of the gas turbine type include a compression section, a combustion section and a turbine section. A flow path for working medium gases extends through the sections of the engine. The working medium gases are compressed in the compression section. The compressed gases are mixed with fuel in the combustion section and burned to add energy to the gases. The hot pressurized gases are expanded through the turbine section.
In the turbine section, the engine is provided with a rotor assembly. The rotor assembly includes a rotor disk and rotor blades which extend outwardly across the working medium flow path. The rotor assembly extracts energy from the gases as the gases pass through the rotor section.
A stator assembly extends circumferentially about the rotor assembly. The stator assembly supports the rotor assembly and provides a pressure vessel to confine the working medium gases to the working medium flow path. Typically, the stator assembly includes an outer case and an inner case which surrounds the hot working medium gases. The inner case supports the arrays of stator vanes and sealing elements such as inner air seals and outer air seals which are in close proximity to the rotor assembly.
As the gases are flowed through the stator assembly, the gases exert aerodynamic loads on the stator vanes. The loads are transmitted from the stator vanes to the inner case causing stresses in the inner case. In addition, the outer air seals and stator vanes are heated by the working medium gases as the gases pass through and over these elements. The heat is transmitted by conduction to the inner case causing uneven heating of the inner case and additional stresses in the inner case which may adversely affect the fatigue life of the inner case.
Cooling air is supplied to the turbine section to cool the inner case. Cooling the inner case decreases thermal gradients in the inner case and the associated thermal stresses in the inner case.
One type of construction for providing a coolable inner case includes circumferentially continuous outer case and a circumferentially segmented inner case. Each segment carries a portion of the arrays of stator vanes and outer air seals. This has the advantage of decreasing thermal stresses in the inner case by interrupting the circumferential continuity of the inner case. But a segmented construction has the disadvantage of requiring a supply of cooling air be at a pressure which is higher than the gas path pressure to avoid hot working medium gases from leaking radially outwardly past the inner case. Once past the inner case, the hot working medium gases have the potential to cause unusually high temperature gradients in the adjacent structure with accompanying high thermal stresses and decreased thermal fatigue life. Leakage of cooling air into the flow path occurs at lower pressure regions of the flow path. Because the high pressure gases must be at a pressure which is higher than the highest pressures in the gas path adjacent the inner case. In addition, these higher pressures require greater amounts of work to compress the cooling gases. The work required to compress the cooling gases reduces the efficiency of the rotary machine.
Another approach is to form the engine of a plurality of circumferentially continuous cases which are joined to each other and which are used to supply the cooling air to the working medium flow path. An example of such a construction is shown in U.S. Pat. No. 4,841,726 entitled "Gas Turbine Jet Engine of Multi-Shaft Double-Flow Construction" issued to Burkhardt. Because the cases are circumferentially continuous, the gases may be supplied at a lower pressure as long as the pressure of the gases is higher than the discharge location adjacent to the hot working medium flow path. This approach increases engine efficiency by decreasing the work of pressurization but results in significant thermal stresses in the inner case as the cases attempt to expand against the structures to which they are rigidly attached.
Accordingly, scientists and engineers working under the direction of Applicants' Assignee have sought to develop a stator assembly having cases which enable the supply of cooling air at pressures lower than the highest pressure in the working medium flow path but avoiding the stresses associated with differences in thermal growth between the inner case and the outer case.